Recently, a demand for mobile communication devices, such as a portable telephone and PHS, is increased with the progress of mobile communication technology. In the case of PHS, several millions of terminal sets have been sold within a year from the beginning of PHS availability. Therefore, hundreds of thousand of base stations for PHS terminals will be needed in one sales company.
In the PHS system, a working frequency band of 1.9 GHz, a carrier interval of 300 KHz, an access manner of multi-carrier TDMA/TDD (time division multiple access/time division duplexing), a modulation manner of .pi./4 shift QPSK (quadrature phase shift keying), a transmission speed of 384 kbps (voice encoding speed of 32 kbps-ADPCM), an occupied frequency band of 280 kHz, and a terminal aerial power of 10 mW are used. Also, one transmission-reception wave of four-channel multiplex TDMA/TDD uses a same frequency, and therefore one TMDA/TDD frame (5 ms, 240 bits.times.8) is sequentially composed of four transmission slots (0.625 ms, 240 bits, 384 kbps) and four reception slots to a base station, where one slot is used as a control slot and the base station conducts the transmission and reception while using the other slots to three terminal stations. The transmission or reception between base station and terminal station is connected through a local exchange network or an exchange to a telephone exchange network, thereby allowing the communication between PHS terminals or telephone and PHS terminal. Due to the above specification, a paired band for transmission and reception is not necessary, therefore facilitating frequency management. Also, a diplexer is not necessary, therefore enabling miniaturization. Furthermore, transmission and reception can be conducted in a same fading state, and only the base station can control the diversity.
However, in the conventional transmitting diversity circuit, there is a problem that the consumed power of a power amplifier is increased. This is because a synthesizer disposed between the power amplifier and an antenna causes a passing loss of about 3 dB and therefore the power amplifier is needed to excessively increase its output level.
Also, there is another problem that the cost of the power amplifier is increased. This is because a transistor with an increased chip size is needed to excessively increase its output level.